1. Field of the Invention
The present invention relates to a load control device for a light-emitting diode (LED) light source, and more particularly, to an LED driver for controlling the intensity of an LED light source.
2. Description of the Related Art
Light-emitting diode (LED) light sources (i.e., LED light engines) are often used in place of or as replacements for conventional incandescent, fluorescent, or halogen lamps, and the like. LED light sources may comprise a plurality of light-emitting diodes mounted on a single structure and provided in a suitable housing. LED light sources are typically more efficient and provide longer operational lives as compared to incandescent, fluorescent, and halogen lamps. In order to illuminate properly, an LED driver control device (i.e., an LED driver) must be coupled between an alternating-current (AC) source and the LED light source for regulating the power supplied to the LED light source. The LED driver may regulate either the voltage provided to the LED light source to a particular value, the current supplied to the LED light source to a specific peak current value, or may regulate both the current and voltage.
LED light sources are typically rated to be driven via one of two different control techniques: a current load control technique or a voltage load control technique. An LED light source that is rated for the current load control technique is also characterized by a rated current (e.g., approximately 350 milliamps) to which the peak magnitude of the current through the LED light source should be regulated to ensure that the LED light source is illuminated to the appropriate intensity and color. In contrast, an LED light source that is rated for the voltage load control technique is characterized by a rated voltage (e.g., approximately 15 volts) to which the voltage across the LED light source should be regulated to ensure proper operation of the LED light source. Typically, each string of LEDs in an LED light source rated for the voltage load control technique includes a current balance regulation element to ensure that each of the parallel legs has the same impedance so that the same current is drawn in each parallel string.
It is known that the light output of an LED light source can be dimmed. Different methods of dimming LEDs include a pulse-width modulation (PWM) technique and a constant current reduction (CCR) technique. Pulse-width modulation dimming can be used for LED light sources that are controlled in either a current or voltage load control mode. In pulse-width modulation dimming, a pulsed signal with a varying duty cycle is supplied to the LED light source. If an LED light source is being controlled using the current load control technique, the peak current supplied to the LED light source is kept constant during an on time of the duty cycle of the pulsed signal. However, as the duty cycle of the pulsed signal varies, the average current supplied to the LED light source also varies, thereby varying the intensity of the light output of the LED light source. If the LED light source is being controlled using the voltage load control technique, the voltage supplied to the LED light source is kept constant during the on time of the duty cycle of the pulsed signal in order to achieve the desired target voltage level, and the duty cycle of the load voltage is varied in order to adjust the intensity of the light output. Constant current reduction dimming is typically only used when an LED light source is being controlled using the current load control technique. In constant current reduction dimming, current is continuously provided to the LED light source, however, the DC magnitude of the current provided to the LED light source is varied to thus adjust the intensity of the light output. Examples of LED drivers are described in greater detail in commonly-assigned U.S. patent application Ser. No. 12/813,908, filed Jun. 11, 2010, and U.S. patent application Ser. No. 13/416,741, filed Mar. 9, 2012, both entitled LOAD CONTROL DEVICE FOR A LIGHT-EMITTING DIODE LIGHT SOURCE, the entire disclosures of which are hereby incorporated by reference.
In addition, some LED light sources comprise forward converters for driving the LED light sources to control the load current conducted through the LED light source. Forward converters comprise a transformer having a primary winding coupled to at least one semiconductor switch and a secondary winding operatively coupled to the LED light source. The semiconductor switch is rendered conductive and non-conductive to conduct a primary current through the primary winding and to thus transfer power to the secondary winding of the transformer. Forward converters typically comprise an optocoupler for coupling a feedback signal on the secondary side of the transformer to the primary side of the transformer, such that a controller can control the semiconductor switch is response to the feedback signal. However, there is a need for a forward converter that is able to control the magnitude of the load current through an LED light source without the need for an optocoupler.